As a type of important polymer product, synthetic latex has ten million tons of annual output, and is widely used in various industrial fields including construction, textile, papermaking, leather, paint and the like. Synthetic latex is a product obtained from polymerization of monomers which is dispersed into an emulsion in water under the action of a surfactant (emulsifier). With the development of theoretical research on emulsion polymerization, and the progress of industrial technology in emulsion polymerization, many new methods of emulsion polymerization emerge, and meanwhile the application field of synthetic latex is also broadened. The use of water, which is a non-toxic and cheap solvent, as the medium imparts unique advantages to synthetic latex, but the presence of a large amount of water undoubtedly increases the cost of transportation and storage of the latex, and prolongs the drying time of the latex. Stability is one of focuses in the study of synthetic latex. It is controlled by the surfactant existed on the interface between polymer particles and water. Conventional surfactants take a good stabilizing effect during emulsion polymerization and storage of latex. However, it is very difficult to destroy such stability or to recover it after destruction. Chinese patent CN101550218 discloses a preparation method of re-dispersible latex powder, and concretely discloses that a polymer emulsion is prepared by emulsion polymerization, an anti-caking agent is then added, thereafter, the emulsion is subjected to spray drying to obtain a re-dispersible latex powder; the preparation process can be carried out without the addition of protective colloid, thus obviously improving the water resistance and stability of the latex powder and leading the latex to have the same properties with the original emulsion when being re-dispersed in water. The prepared re-dispersible polymer latex powder has high solid content (approximately 100%), can reduce the difficulties of delivery and storage, can be packed by paper, reduce packing cost, has good storage stability and long storage period and is not deteriorated due to freezing or volatilization of water. Chinese patent CN101445574 discloses a core-shell polymer emulsion for manufacturing re-dispersible latex powder and preparation method thereof, the preparation method comprising the steps of preparing monomer pre-emulsified solution to form the core, preparing monomer pre-emulsified solution to form the shell and preparing core-shell polymer emulsion in sequence. The core-shell polymer emulsion obtained according to the invention has excellent film-forming water resistance and tensile strength, and can be applied to various fields including coating industry, chemical building materials, resource utilization of industrial solid wastes, and the like. Chinese Patent CN1217733 discloses a preparation method of polymer powder re-dispersible in aqueous solution, the polymer powder containing free acid or basic group-bearing copolymers, wherein a buffer agent is added before drying to adjust the pH so as to achieve the re-dispersion of the latex. Chinese Patent CN1325921 discloses a process for preparing a re-dispersible synthetic resin powder which adsorbs polyvinyl alcohol resin on its surface, which powder has excellent re-dispersibility, and can be easily dispersed into water to form emulsion. Chinese patent CN101575399 discloses a preparation method of an acrylic acid re-dispersible latex powder, which comprises preparing an acrylic acid re-dispersible latex powder emulsion and spray drying of the acrylic acid re-dispersible latex powder emulsion. The acrylic acid re-dispersible latex powder emulsion is designed in the form of core-shell particles and is obtained by emulsion polymerization of acrylic acid monomer. In the course of spray drying the emulsion, a mineral anti-adhesive agent is added, to thereby obtain the acrylic acid re-dispersible latex powder. The acrylic acid re-dispersible latex powder can be added in concrete and mortar to improve the compressive strength, rupture strength, wear resistance, toughness, adhesion strength, water-retention capacity and constructability of the concrete and mortar.
Recently, Zhao et al published a research report “General Strategy for Making CO2-Switchable Polymers” in ACS Macro Lett 2012, 1:57-61. They found that, by switching between simple, environmentally friendly trigger conditions—carbon dioxide and nitrogen, it is possible to reversibly regulate the lowest critical solubility temperature (LCST) of poly(dimethylaminoethyl methacrylate) (PDMAEMA): tertiary amino group of PDMAEMA can react with carbon dioxide in water to form bicarbonate which makes it possible to greatly enhance the LCST; after the removal of carbon dioxide by using nitrogen, argon or air, the PDMAEMA can recover to its original state. They prepared reversibly swellable and shrinkable hydrogel based on the stimuli-response of PDMAEMA to carbon dioxide. In addition, Wang et al published a research report “Reversibly Coagulable and Re-dispersible Polystyrene Latex Prepared by Emulsion Polymerization of Styrene Containing Switchable Amidine” in Macromolecules 2011, 44:6539-6545. They prepared reversibly coagulable and re-dispersible polymer latex through soap-free emulsion polymerization by using amidine-containing compounds as comonomers with switchable surface activity. However, the latex thus prepared can't be demulsified through charging nitrogen.